Mattress spring core construction over the years has been a continuously improving art with advancements in materials and machine technology. A well-known form of spring core construction is known as a Marshall spring construction wherein metal coil springs are encapsulated in individual pockets of fabric and formed as elongate or continuous strings of pocketed coil springs. In an earlier form, these strings of coil springs were manufactured by folding an elongate piece of fabric in half lengthwise to form two plies of fabric and stitching transverse and longitudinal seams to join the plies of fabric to define pockets within which the springs were enveloped.
Improvements in spring core constructions have involved the use of fabrics, which are thermally or ultrasonically weldable to themselves. One such cost-effective fabric is a spun-bonded polypropylene fabric. By using such welding techniques, these fabrics have been advantageously used to create strings of individually pocketed coil springs wherein transverse and longitudinal welds, instead of stitching, are used to form the pockets encapsulating the springs.
Once strings of pocketed springs are constructed, they may be assembled to form a spring core construction for a mattress, cushion or the like by a variety of methods. For example, multiple or continuous strings may be arranged in a row pattern corresponding to the desired size and shape of a mattress or the like, and adjacent rows of strings may be interconnected by a variety of methods. The result is a unitary assembly of pocketed coil springs serving as a complete spring core assembly.
A pocketed spring assembly may be surrounded with a border made of foam or any other suitable material to provide edge support around the perimeter of the pocketed spring assembly. Such a pocketed spring assembly is mounted upon a base and is completely enclosed within an upholstered covering material. The base and border are known in the industry as a “bucket” into which a pocketed spring assembly may be inserted before the “bucket” is covered with one or more padding or cushioning layers. Upon receiving multiple pocketed spring assemblies, a mattress manufacturer must insert each of the pocketed spring assemblies inside a bucket specifically constructed to receive a specified size of pocketed spring assembly. The mattress manufacturer must construct the foam encasements or “buckets” of different sizes via separate processes, which have proven to be costly due to the labor cost required.
Mattress manufacturers would prefer to eliminate the process of building foam encasements or “buckets” and instead receive a pocketed spring assembly within built-in edge supports along all four sides of the pocketed spring assembly. It is generally known within the bedding industry that edge supports made of pocketed springs are more durable than foam edge supports. Pocketed spring assemblies having pocketed spring edge supports may be roll packed for shipping whereas those having foam edge supports are not easily roll packed for shipping.
Pocketed spring assemblies made by joining parallel strings of individually pocketed springs have been made with four sides of edge support due to pocket coil machines capable of changing the springs within a strand or string of individually pocketed springs. Such modern pocket coil machines may further create posturized pocketed spring assemblies with zones or areas of different firmness.
While modern pocket coil machines may change springs “on the fly”, the springs being individually pocketed to create edge support, the cavities into which the different springs are inserted before being closed to create a pocket are the same size. Therefore, the pockets within a string are the same size prior to insertion of the springs regardless of which springs are inserted therein. Because spun-bonded polypropylene fabric used in the strings may stretch, over time, some of the pockets may stretch to a different dimension than other pockets within a string due to different coil springs having different geometries which may exert different degrees of force on the spun-bonded polypropylene fabric of the pockets. This stretching may result in the pocketed spring assembly having an uneven surface which is not desirable.
Coil springs of one region of a pocketed spring assembly which are firmer than coil springs of another region may undesirably create what is referred to in the industry as a “step”. For example, coil springs around the perimeter of a pocketed spring assembly which are firmer than the core or interior springs may undesirably create four “steps”. A pocketed spring assembly having such a four-sided “step” have displayed the undesirable appearance of sagging towards the middle of the pocketed spring assembly.
Currently, to avoid a “step” or an uneven surface, a mattress manufacturer, for example, may use coil springs less firm than ideal around the perimeter of the pocketed spring assembly and/or coil springs softer than ideal for the center or core of the pocketed spring assembly. Stated differently, a pocketed spring assembly may be manufactured with firmness differential which is less than possible with modern pocket coil machines. The present invention provides a pocketed spring assembly having different zones or regions of desired different firmness without “step”. The present invention provides pockets of different dimensions or sizes along a strand or string to accommodate different geometries of the coil springs.
Therefore, there is a need for a pocketed spring assembly lacking any step made of strings with spun-bonded polypropylene fabric which have different zones or regions of different firmness due to different springs within the pockets.
There is further a need for a posturized pocketed spring assembly with a generally smooth upper surface, which when received by a mattress manufacturer, does not require additional edge support to be added, thereby reducing the cost of manufacturing a finished mattress.